The present invention relates generally to a technology of calculating an exposure dose and a focus position in an exposure apparatus.
The pattern exposure technology that transfers an LSI pattern on a reticle onto a wafer is required to improve the productivity as well as promoting the fine processing to a transferable pattern. The exposure process in the exposure apparatus sets an optimal value of an exposure condition, such as a focus position and an exposure dose, for each process and for each exposure layer, in exposing the wafer.
The optimal exposure condition, such as the focus position and the exposure dose, is set as a result of that a shape measurement apparatus, such as a critical dimension scanning electron microscope (“CD-SEM”) measures a predetermined resist pattern shape. An etched pattern shape is also measured, and both shapes are compared. However, various errors, such as the resist's refractive index and thickness, exposure control and focusing of the exposure apparatus, a development time period, a developer's characteristic, an uneven hot plate, a PEB temperature and time period, and a reticle manufacturing error (flatness), differentiate their measurement values, cause a CD error, and lower a manufacturing yield rate. One proposed method calculates and controls the exposure dose using a means for setting a resist's refractive index and thickness. See, for example, Japanese Patent Application, Publication No. 62-132318. Another proposal improves the wafer's CD error by adjusting the exposure dose against the reticle error. See, for example, Japanese Patent Application, Publication No. 10-032160. When the CD fluctuates approximately concentrically to the wafer, still another proposal prepares a function between a distance from a wafer's center and the exposure dose, and controls the exposure dose for each shot. See, for example, Japanese Patent Application, Publication No. 10-064801. Yet another proposed method prepares an exposure dose map for each shot from CD measurement values of the exposed whole wafer plane and exposed whole shot plane, and controls the exposure dose in accordance with the map. See, for example, Japanese Patent Application, Publication No. 2005-094015.
The fine-processing demand gradually increases a numerical aperture (“NA”) of a projection optical system in the exposure apparatus, and gradually decreases a depth of focus (“DOF”). For example, the DOF for the 90 nm CD pattern is merely about 200 nm with 10% latitude for the CD fluctuation.
A method of setting the exposure condition for the narrow DOF is required to change a focus position to an exposure dose window (“ED window”) as well as control over a CD value through a change of an exposure dose.
Japanese Patent Applications, Publication Nos. 62-132318, 10-032160, 10-064801, and 2005-094015 propose methods of setting an exposure dose suitable for each of various errors, but are silent about an effective control method of controlling a focusing position. This is because no prior art provide exposures with minutely changed focus positions unlike a focus exposure matrix (FEM) pattern, or an approach that calculate a shift amount of a focus position from one exposure condition. One reason rests in use of a CD-SEM for a CD measurement. The CD measurement from one mark provides only one measurement result of a CD value, and only one variable can be varied. Therefore, Japanese Patent Applications, Publication Nos. 62-132318, 10-032160, 10-064801, and 2005-094015 has limits to the transferable pattern. Accordingly, one proposed method calculates an offset of an exposure dose and an offset of a focus position (substrate position) in the exposure apparatus. See, for example, Japanese Patent Application, Publication No. 2003-142397, which creates a library of a correlation among the offsets of the exposure dose and the focus position (substrate position) and a light intensity signal waveform of a pattern, and measures the light intensity signal waveform of the pattern obtained for a certain exposure dose and focus position. Thereafter, this reference calculates the offsets of the exposure dose and the focus position based on the library and the measured light intensity signal waveform.
Nevertheless, the previously prepared library does not conform to the status quo in the method of Japanese Patent Application, Publication No. 2003-142397 as an extrinsic factor that affects the CD fluctuates, such as the resist's refractive index and thickness, exposure control and focusing in the exposure apparatus, a development time period, a developer's characteristic, an uneven hot plate, a PEB temperature and time period, and a reticle manufacturing error (flatness). Then, this method needs to properly update the library.